1. Field of the Invention
The invention relates to an electric supercharger and more particularly, relates to an electric supercharger in which an electric motor (rotating electrical machine) that rotates the supercharger is cooled.
2. Description of the Related Art
A supercharger that compresses air to be supplied into an engine by rotation of a compressor wheel and supplies the compressed air to the engine in order to improve the output of the engine, is known. Also, an electric supercharger equipped with a rotating electrical machine that applies torque to the compressor wheel, is known. For example, Japanese Patent Application Publication No. 2005-90403 (hereinafter referred to as “JP-A-2005-90403”) discloses a lubrication control device of an electric supercharger.
The lubrication control device of the electric motor-equipped supercharger disclosed in JP-A-2005-90403 lubricates and cools a rotating shaft of a turbine and compressor by using lubricating oil that is circulated in a turbo unit. The lubrication control device sets the amount of lubricating oil with the drive rotation speed of a motor that is provided in the supercharger. As a result, oil may be prevented from running out of and a reliable lubrication may be realized by increasing the amount of lubricating oil when the motor provided in the supercharger rotates at a high speed. Also, the turbo unit may be cooled sufficiently because a larger amount of heat is carried away from the turbo unit. On the other hand, when the motor provided in the supercharger rotates at a low speed, an increase in rotational resistance due to viscosity of the lubricating oil may be restrained by reducing the amount of lubricating oil within a necessary range that amounts of lubrication and cooling are secured.
Incidentally, Japanese Patent Application Publication No. 2003-102147 (hereinafter referred to as “JP-A-2003-102147”) discloses a cooling device for a general motor that is provided with a stator-side cooling passage that cools the stator of the motor and a rotor-side cooling passage that cools the rotor. The cooling device further includes distribution means that distributes a cooling liquid (refrigerant) between the stator-side cooling passage and the rotor-side cooling passage in accordance with an arbitrary flow ratio. The distribution means changes the flow ratio between the stator-side cooling passage and the rotor-side cooling passage according to the rotor rotation speed. As a result, a cooling efficiency of the cooling device may be improved taking into account the fact that the heat generation characteristics of the rotor and the stator change according to the rotation speed.
However, the temperature rise characteristics of the rotor of the motor provided in the electric supercharger is not determined only by the rotor rotation speed, but is also greatly affected by the temperature of exhaust gas that rotates a turbine wheel connected to the motor. For example, when the engine is operated at a low rotation speed with high load, the rotor temperature of the motor does not rise very high because the temperature of exhaust gas from the engine is low although the rotor rotation speed rises.
According to the disclosures of JP-A-2005-90403 and JP-A-2003-102147, the supply amount of the refrigerant (lubricating oil) is controlled only focusing on the rotation speed (rotor rotation speed) of the motor provided on the supercharger. Therefore, the motor is incapable of cooling down efficiently because a large amount of lubricating oil is supplied to the motor even when the rotor is in low need for cooling down, for example, even when the engine is operating at a low rotation speed with high load. As a result, an energy consumption of the lubricating oil supply mechanism such as the oil pump, unnecessarily increases, and the fuel economy of the vehicle becomes worse. Furthermore, if lubricating oil is excessively supplied to the motor, the rotational resistance of the rotor is unnecessarily increased. From this standpoint, the fuel economy of the vehicle also becomes worse.